Industrial requirements often result in the use of multi-burner boiler and industrial furnace installations that utilize a fossil fuel like pulverized coal for fuel and utilize an oil pilot flame. Neither a lead sulfide nor a silicon detector used separately have been able to sense and discriminate satisfactorily between safe and unsafe burner operation in these multi-burner arrangements. A system utilizing both a lead sulfide (PbS) detector and a silicon (Si) detector is effective to combine the best characteristics of both detectors to provide satisfactory flame sensing. More specifically the coal flame sensor of this invention has shown the capability of providing flame recognition with discrimination between supervised burner, adjacent burners and the background fireball for boilers fueled with pulverized coal at widely varying loads. While specific reference is made in this specification to pulverized coal for fuel and utilizing an oil pilot torch to ignite the coal, it should be noted that in addition to pulverized coal fuel boilers, the present invention is also useful with other fuels such as waste fuel fired boilers, in the pulp and paper industry liquor recovery boilers, and in heavy oil burners as well.
As stated above in order to sense the flame condition more accurately two dissimilar sensors are used, one of which is made of Si and the other is PbS. In a simplified explanation of the operation of the system it may be said that the flames of the multi-burner array all characteristically have both a DC and an AC component in radiation intensity. The AC components of the various flames in the boiler generally cancel out, so that the only significant AC component to be observed comes from the nearest flame, i.e. the one under observation. A problem which when using the PbS sensor only is that the high level DC signal from the background fireball tends to reduce the AC or flicker sensitivity of the PbS sensor. The problem when using a Si sensor only is related to the limited range of wavelength to which it is sensitive. The Si sensor is sensitive primarily in the visible region as opposed to the IR (infrared) region where the PbS sensor operates. Due to combined effects of the masking effect of coal dust on visible light and the lower level of the AC signal in that visible region of the spectrum of the flame the pulverized coal will produce very little flicker signal for detection by the Si sensor. The oil pilot flame, however, produces a strong visible flicker and because of its positioning is subject to very little masking. The AC signals from each of the sensors are summed. The DC signal from the Si sensor is used to control the gain applied to the AC signals. The system is arranged with a signal divider so that this gain varies as an inverse function of the DC signal observed by the Si cell. While specific reference is made in this invention to the infrared sensitive detector PbS, it should be noted that other infrared sensors may be used such as lead selenide or germanium detectors either with or without an appropriate optical band-pass filter.
In these multi-burner array furnaces a number of possible situations may arise. If a number of burners other than the burner being observed are operating, there will be a background fireball but the AC component seen by the dual detector of the burner being observed will be substantially non-existent and the alarm will indicate. When the oil fed pilot touch is inserted into the burner being observed, the oil flame provides a strong AC component signal in the visible range which is seen by the Si sensor, so the indicator shows a safe condition. When pulverized coal is then fed to the burner and the main flame (coal) is ignited and burning, the magnitude signals both DC and AC will decrease in visible range but AC will increase in IR range to show a safe condition.
Prior art such as the patent to Wheeler U.S. Pat. No. 3,689,773 entitled "Flame Monitor System and Method Using Multiple Radiation Sensors" shows the use of two identical radiation sensors to simultaneously sense radiation from two different parts of a flame exhibiting high radiation fluctuations. In the present invention, to discriminate the condition of the flame more accurately, two differing or dissimilar sensors one a PbS (1-3 um) and the other a Silicon (0.35-1.1 um) are used.